The present invention relates generally to an electronic timing reference apparatus, and more particularly to a crystal timing apparatus and method that improves and maintains the timing accuracy of a digital reference timing signal for electronic and digital equipment.
Modern military, aerospace and communications systems often require extremely stable and accurate timing devices. The most common timing reference in these electronic systems is a crystal oscillator. However, crystal oscillators are prone to slight changes in frequency. In fact, two major factors contribute to changes in the frequency of a crystal oscillator. The first is temperature and the second is acceleration. To avoid temperature-related drift, prior art precision timing references typically involve embedding a crystal oscillator in a temperature-controlled shell. Because it is easier to heat a device than to cool it, the shell is typically maintained at a temperature above the ambient temperature to thus maintain the crystal well above ambient temperature. Such a temperature-controlled crystal is often referred to as an xe2x80x9covenizedxe2x80x9d crystal.
Often the principal contributor to acceleration is vibration of the circuit board on which the crystal is mounted. One prior art method for reducing this vibration is to mechanically isolate the ovenized crystal from its circuit board, typically by using flexible electrical connections (i.e. wires) as springs. However, this does not completely eliminate vibration. Furthermore, it does not reduce low-frequency or steady-state acceleration, such as the acceleration experienced while a vehicle is rapidly increasing in speed or performing a maneuver.
Timing drift due to low-frequency and steady-state acceleration will be particularly troubling in planned, high-bandwidth mobile military networks. The data rate for a single link in these networks can be 100 Mega bits per second (Mbps) or more. Direct links between nodes may be as long as 900 km. To attain such high data rates at long ranges, directional antennas, such as electronically steered phased array antennas, must be used. Phased array antennas have certain advantages, including the ability to hop the beam from target to target thousands of times per second. Hopping the beam permits many links to be established per antenna, but requires that each link use a multiplexing protocol, such as a Time Division Multiple Access (TDMA) protocol.
For the quality of service needed in these high bandwidth networks, the TDMA time slots must be accurately timed to within two or three microseconds, and this timing precision must be maintained when the Global Positioning Satellite (GPS) system is jammed for half an hour or more. This requires timing drift of less than 2 parts per billion. Timing stability at this level can be easily provided by atomic clocks, which are used by GPS systems, but atomic clocks are too big and too heavy to fit in most missiles, spacecraft, military aircraft or land vehicles. As a result, state-of-the-art ovenized crystal timing references will not meet the requirement of high-performance military TDMA networks.
Accordingly, there remains a need for a small, efficient solution to improve timing with a crystal oscillator as well as reduce the drift rate of ovenized crystal oscillators.
The foregoing drawbacks are overcome by an improved crystal time reference apparatus in accordance with the preferred embodiments of the present invention. The apparatus overcomes the aforementioned disadvantages as well as other disadvantages by reducing the drift rate of ovenized crystal oscillators.
The apparatus is utilized to produce a synchronized timing signal by mounting an accelerometer adjacent to a reference crystal for accurately sensing acceleration experienced by the reference crystal during use on a mobile platform. An offset generator in communication with the accelerometer converts the acceleration measurements generated by the accelerometer to error correction signals. These signals represent offset values needed to compensate for timing drift of the reference crystal output caused by the acceleration forces it experiences.
Upon receiving an output of the reference crystal and an output from the offset generator, the system determines a corrected timing reference signal. The present invention thus achieves a smaller and more precise timing device than previously developed time reference devices. Most importantly, the present invention is ideally suited for use on mobile platforms, such as commercial and military aircraft, where space is at a premium and the possibility of using other means to provide a more accurate timing reference, such as an atomic clock, is not a viable option.